Many implantable medical devices include among their operation the sensing and detection of one or more physiological parameters. Such devices may include one or more implantable sensing electrodes and typically include corresponding signal amplifier and sensing circuits. As used herein, “sensing” refers to developing a signal (such as an electrical potential) corresponding to one or more physiological parameters and the sensing can occur continuously and/or on a periodic or sampled basis. “Detection” refers to the determination of the sensed signal as corresponding to the occurrence or lack thereof of a given physiological process.
Some difficulties faced with accurately sensing and detecting physiological processes, such as with an implantable medical device, include that many physiological parameters of interest generate relatively low level base signals and are frequently presented in a background of relatively wide band noise which may be of greater magnitude than the signals of the physiological process of interest. A further difficulty is that a physiological process of interest may be of irregular or intermittent nature thus complicating accurate detection, e.g. whether a given physiological process is absent, delayed, premature, or of smaller magnitude than expected. For one example, implantable cardiac stimulation devices such as pacemakers and/or cardioverter defibrillators (ICDs) automatically sense the patient's cardiac activity by deriving electrical signals from the electrochemical activity of the heart, use these sensed signals to detect the presence/absence, magnitude of, and relative timing of activity in cardiac tissue and automatically supply therapeutic electrical stimulation as indicated to provide desired cardiac rhythm.
In implantable medical devices, such as pacemakers and ICDs, electrical signals picked up by one or more sensing electrode in contact with the patient's cardiac tissue is conducted by one or more leads to battery powered sensing circuitry typically contained within a hermetically sealed biocompatible housing or can which is also implanted within the patient. The sensing circuitry typically includes variable gain amplifier and detection circuitry which evaluates the amplified signals sensed from the patient's cardiac tissue for determination of presence and timing of cardiac activity of interest. A threshold is typically set such that sensing of a cardiac signal within a given time window and above the threshold constitutes “detection” of the corresponding physiological process, such as an atrial or ventricular contraction. The gain of the amplifier and the setting of the threshold is preferably set such that the device accurately senses and detects cardiac activity occurring with reduced incidence of failure to detect such activity (undersensing) as well as reduced incidence of falsely “detecting” signals as corresponding to a physiological process not actually present (oversensing). However, such setting of the gain and thresholds is complicated by the variable timing and magnitudes of the signals of interest particularly in patients for whom implantation of a cardiac stimulation device is indicated.
A variety of devices and algorithms are known which attempt to more accurately determine and set appropriate gain and threshold settings for an implantable medical device and patient combination. Such devices and algorithms may operate automatically within the device itself and/or may operate in a command manner via an external device in communication with the implantable device, such as a physician's programmer. Setting and evaluation of amplifier gains and thresholds may occur at initial implantation as well as periodically throughout the implant lifetime.
For example, one process for setting the sensing thresholds involves establishing communication between an implantable cardiac stimulation device and an external physician's programmer. A clinician then commands, via the physician's programmer, the implantable device to progressively program itself to less sensitive settings. The performance of the implantable device with the progressively less sensitive settings is evaluated with respect to independent measurement of the patient's cardiac activity via a surface electrocardiogram (ECG) and loss of sensing is recognized by either competition between intrinsic and paced activity or failure of the device to indicate an appropriate sense marker despite visible observation of the native signal by the surface ECG. The clinician would terminate the test and use this data to then program the most appropriate threshold settings. A refinement of this general process is also known wherein, upon initiation of the test, the test proceeds automatically with communication between the programmer and the implantable device. However, these processes are subject to misinterpretation leading to instances of suspect results which will be discussed in greater detail below.
One theoretically possible remedy for more accurately conducting sensing measurement tests, such as sensing of the P and R waves of a patient's cardiac cycle, is to perform relatively high frequency sampling of the wave forms to much more accurately obtain information relating to the magnitude and timing of these physiological signals. However, in practical application, this has not proven a viable possibility. Firstly, the microprocessor and memory resources of the implantable device are limited in capacity and the allocation of the necessary processing resources to support very high rate sampling has not proven feasible. Secondly, relatively high rate sampling is also relatively demanding of the limited energy capacity of the battery and thus employing high rate sampling, particularly on a periodic basis to re-verify or adjust sensing threshold throughout the implantation period unacceptably drains the battery indicating an early invasive procedure to replace the battery.
Thus, from the forgoing it will be appreciated that there is a need for a system that can more accurately determine appropriate sensing threshold parameters in an implantable medical device, such as in an implantable cardiac stimulation device, to facilitate more accurate detection of physiological processes of interest. There is a particular need for a system of setting such sensing threshold parameters in applications where the subjects' physiological processes generate indicia that are irregular in nature over time. It would be further advantageous for such a system to be provided in a manner that is substantially automated to reduce operating burden on clinicians or other users of the system.